Optical sensors for determining the carbon dioxide content of the air are used, among other things, in fire detectors. Their function is based on the fact that a layer sensitive to carbon dioxide changes color reversibly upon contact with the gas to be determined. This change of color is monitored by a detector, and when a predetermined minimum concentration is exceeded, an alarm is triggered. This measuring method is relatively susceptible to contamination.
Optical CO2 gas sensors normally are made up of a polymeric matrix, e.g. ethylcellulose, a softener and solutizer, e.g. Brij 30, and a basic gas acceptor, e.g. tetraoctylammonium hydroxide. The sensor signal reveals a dependency on the moisture content of the surroundings.
U.S. Pat. No. 6,241,873 describes a carbon dioxide sensor which detects the carbon dioxide content of a surrounding atmosphere in a potentiometric manner. It features a measuring electrode and a reference electrode, which are applied on a substrate. The measuring electrode takes the form of a silver/silver carbonate electrode. The potential of this electrode is a direct function of the carbon dioxide concentration of the surroundings. A disadvantage of this measuring method is the fact that carbonate-containing electrodes are affected by weather influences and thus have only a low stability. Furthermore, the sensor is limited to measuring carbon dioxide.
The present invention is based on the objective of providing a gas sensor for determining different gases in a potentiometric and/or optical manner, which has a high stability and at the same time a high sensitivity.
A further objective lies in the compensation of the moisture-dependency of the sensor.